Altitude sickness

Written by Dr Charlie Easmon, specialist adviser in travel medicine

What is altitude sickness?

Altitude sickness is the name given to the physiological and symptomatic reactions of the human body to the low oxygen pressure ('thinner' air) that occurs at high altitude. It is also known as acute mountain sickness.

Where do you get altitude sickness?

About 20 per cent of people experience mild symptoms at altitudes between 2200 and 2500m above sea level. Many ski resorts are found at these altitudes.

The incidence of altitude sickness, which varies from one individual to another, is directly related to the rate of ascent. It is also significantly related to how long a person stays at that height.

The risk of getting altitude sickness in areas such as Nepal and the Andes mountains, where tourist regions can be as high as 3000-4000m, is very real. Each year, out of 50,000 travellers to Nepal, there are at least seven altitude-related deaths. The death rate is about 4 per cent for trips to peaks above 7000m.

How do you get altitude sickness?

When oxygen pressure falls, the body puts in place a number of counter measures. Breathing and pulse rates increase, as does the heart's pumping efficiency, and the size and number of red blood cells, which are responsible for the blood's oxygen carrying capacity.

However, the shortage of oxygen also has a number of unwanted consequences: increased pressure in the pulmonary (lung) circulation, changes in blood pH (acidity) values, disturbances in the fluid/electrolyte (salt) balance, as well as the leakage and spread of blood or fluid into surrounding vessels and tissues (fluid extravasation or oedema).

Altitude sickness can occur by travelling relatively quickly to a greater height and staying at the new height for more than 24 hours, without adequate acclimatisation beforehand. Altitude sickness is not dependent on a person's fitness and can affect even the most experienced athletes.

How can altitude sickness be prevented?

By taking a graded ascent. Climb relatively slowly to higher levels, and allow adequate periods of acclimatisation (two to three days) at a given height (starting from 2200m) before spending a night at a greater height.

It's fine to climb up during the day, but you should try to get down to 2200m (or the height you are currently acclimatised to) in the course of the same day. Then you can move up, depending on your individual tolerance, by 300 to 500m, until you rest and get acclimatised again for at least a couple of days, and so on.

If you feel ill at a particular height, come down to your previously acclimatised height.

Drink plenty of liquids (at least three litres a day). Avoid drinking alcohol.

Avoid getting cold.

To a certain extent, altitude sickness can be prevented by taking 750mg acetazolamide per day from one day before ascent until two days after reaching the maximum height.

Some experts suggest that to get to know the possible drug side effects it is wise to give it a two-day trial before the trip. This is an unlicensed use of this medicine, which is also only available on prescription, so it should only be undertaken on the advice of a doctor.

Possible side effects include: nausea taste disturbance, tingling hands and feet, frequent and copious urination, visual disturbances and skin rash. However, taking Diamox does not mean people can ignore advice about slow ascent.

Danger signals for altitude sickness

Danger signals usually develop in the first 36 hours. They affect more than 50 per cent of travellers above 3500m and almost 100 per cent of people who climb quickly to 5000m without acclimatising.

An insignificant headache that disappears with one to two ordinary headache tablets.

Nausea and general malaise.

Slight dizziness.

Some difficulty sleeping.

With these symptoms at heights below 3000m, you can usually allow yourself to stay on and to rest for a couple of days before further permanent ascents. At heights around 3500m, you should try moving down 300 to 500m and stay there for two days before further permanent ascents.

Serious symptoms of altitude sickness

A severe, enduring headache, which is not cured by a couple of ordinary painkillers.

Marked nausea and repeated vomiting.

Irritating dizziness or actual difficulty with balance and direction.

Regular visual disturbances with flickering vision and problems judging distance.

Pressure in the chest, rapid breathing and pulse rate, crackles in breathing and shortness of breath.

Swelling beneath the skin (oedema), typically around the eyes and in some cases swollen ankles and hands.

Confusion.

Convulsions.

In the presence of these symptoms, medical attention must be sought immediately, and the patient brought down to the lowest possible height.

Acute mountain sickness

Acute mountain sickness is the name given to two life-threatening complications of acute altitude sickness. Both have a high mortality rate and may occur when you have already spent 24 to 36 hours at 'too great' a height (typically over 3500m). These are:

HAPE - high altitude pulmonary oedema (water in the lungs).

HACE - high altitude cerebral oedema (fluid on the brain).

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